Vehicle wheel bearings generally consist of a spindle surrounded by a generally cylindrical hub and held apart by a double row of rolling elements, such as bearing balls. Each end of the annular space that contains the balls must be sealed, both to retain lubricant and to exclude outside contaminants. The seals also must be able to stand up to the internal pressure fluctuations that the bearing experiences. The bearing space heats up with operation, especially due to brake waste heat, which tends to force air and lubricant out. As it cools down, it can experience a pressure depression, which tends to draw air and contaminants in from outside.
With trailing, nonpowered wheels, the spindle need not extend very far past the end of the hub, unlike the central drive shaft of a powered wheel. Therefore, the seal at the inboard side of the space can be an end cap, which does not rub on the spindle or create any seal friction. One drawback to a cap, however, is that its large surface area, compared to a smaller rubbing seal, can be strongly acted on by an internal pressure fluctuation. This is even more of a problem for a plastic end cap, which cannot be press or friction fitted into a hub with the same force that a metal cap can. Excess pressure could potentially blow it out, and negative pressure could bow it in. Vented plastic end caps are known. However, room for improvement exists in terms of simplicity and compactness, and in terms of protecting the vent from damage and from outside contaminants.